The present invention relates generally to testing visual acuity, and in particular, to an apparatus and method for accurately evaluating night vision after refractive surgical procedures.
It is well known that many people have difficulty seeing at night or during conditions of low ambient illumination. Those who engage in activities such as night driving or flying must be tested to determine visual acuity under simulated nighttime conditions. The prior art is replete with various devices and methods directed to that end.
An example can be found in U.S. Pat. No. 4,764,007 to Task, which relates to a glare susceptibility tester in which an acuity target having translucent recognizable indicia on an opaque background or opaque indicia on a translucent background, is transilluminated and presented to a person being tested with a side-by-side glare source for simultaneous viewing. The observed glare obscures a portion of the pattern of the acuity target to a degree corresponding to the person's susceptibility to glare. Visual function is determined for each of the targets with and without the glare source being selectively exposed such that diminution of visual performance as a result of glare can be determined.
U.S. Pat. No. 4,403,842 to Aulhorn et al. is directed to an apparatus for testing twilight vision and blinding sensitivity. The person being tested looks inside a housing to view a screen having a test figure which is illuminated by projection and a headlight simulating source disposed adjacent to the field of vision. The test figure includes indicia disposed on an outer field wherein the contrast between the outer field and the indicia may be varied to test twilight vision. Blinding sensitivity is tested by turning on the headlight source for short periods of time.
U.S. Pat. No. 3,684,355 to Molner is also directed to a method and apparatus for testing glare susceptibility where the person being tested views both a representative target contained on a film strip which is variably illuminated and a glare source of variable intensity.
U.S. Pat. No. 4,412,729 to Hartmann teaches a test field for providing test conditions resembling those to which the eye is subjected to when working with an electronic CRT terminal. The test field contains indicia such as characters or symbols which appear bright on a darker background and brighter areas of luminance on the order of five to ten times greater than the luminance of the indicia.
The prior art vision testing devices and methods such as those noted above have been found to be unsuitable for fully evaluating night vision after refractive surgical procedures. When refractive surgery is performed on the eye, a central portion of the cornea less than the entire structure is modified, thereby reducing the effective optical area. Existing night vision tests, especially those developed for testing glare susceptibility, typically provide an overall level of illumination sufficient to cause the pupil to become small or moderately sized. While this can be advantageous in replicating actual conditions such as, for example, those encountered during exposure to headlights when driving at night, there are also nighttime conditions under which the ambient illumination is sufficiently low to cause the pupil to become large. Thus, after refractive surgical procedures, a small pupil under test conditions can render determination of visual function inaccurate because under actual nighttime conditions the true pupil size may be large enough to include portions of the cornea which may not have been modified to the new optical state.
It is therefore an object of the present invention to provide a method and apparatus for measuring visual function under simulated nighttime conditions when the pupil is large.
It is yet another object of the present invention to provide a method for measuring visual function under simulated nighttime conditions where the test field is placed at the far field of the eye to minimize pupillary constriction.
It is yet another object of the present invention to provide an apparatus for measuring night visual function under simulated nighttime conditions in which a test field provides a bright resolution acuity target while maintaining extremely low ambient illumination.
It is still another object of the present invention to provide an apparatus and method for measuring visual function under simulated nighttime conditions where the cone of light entering the eye may be artificially limited to ascertain the true functional optical zone of the cornea.